factor Xa and urea?

Hi, I have a fusion protein with maltose-binding protein and I seem to have
trouble in cutting off the MBP part with factor Xa. I have heared that other
people have this problem, and that it can be due to poor accesibility of the
cleavage site. One way of improving cleavage would be to (partially) unfold the
fusion protein, e.g. with urea. Of course, factor Xa would still have to be
active.
Does anyone out there have experience with this kind of experiment, and do you
know at what urea concentrations Xa is still active? Any other ideas?
Thanks, Flip